1. Field of the Invention.
This invention relates to a vehicle and more particularly to a materials handling vehicle of the kind (hereinafter referred to as being of the kind specified) comprising an elongate chassis having at least two pairs of ground wheels which are movable by an operator to effect steering of the vehicle, the wheels of each pair being mounted at opposite sides of the chassis, at least one pair of the wheels being lockable in a position such that steering is effected solely by the other pair of wheels.
Such vehicles are known which comprise a front pair of wheels and a rear pair of wheels, the operator having a selector to lock the rear wheels so that two wheel or simple steering is effected solely by the front wheels. Also upon operation of the selector, the rear wheels may be released from the locked condition and coupled to the front wheels so that as a steering wheel or other control is operated is operated, the operator can perform compound steering using all four wheels so that the rear wheels are moved either in the same sense as the front wheels, whereby the vehicle performs what is known as "crabbing" or in an opposite sense to the front wheels so that the vehicle can perform what is known as "cramping".
After crabbing or cramping, when it is desired to return the vehicle to simple two wheel steering, it is necessary for the rear wheels to be lined up so that the wheels are parallel to the longitudinal axis of the chassis, or else the vehicle will not handle properly when in simple two wheel steering operation, and the tyres on the rear wheels particularly may become prematurely worn.
U.S. Pat No. 3,185,245 discloses a vehicle of the kind specified having a control means to enable an operator to select simple or compound steering and to ensure that the lockable wheels are locked in an aligned position when changing from compound to simple steering, and to ensure that the two pairs of wheels moved in phase when changing from simple to compound steering. By "in-phase" we mean that the angles between the front wheels and the longitudinal axis of the vehicle are of generally the same magnitude as the angles between the rear wheels and the longitudinal axis of the chassis.
However in the arrangement shown in U.S. Pat. No. 3,185,245 a selection of simple or compound steering is not entirely at the discretion of the operator. Particularly when an out of phase condition is sensed, the control circuit automatically adopts a simple steering mode until the phase is corrected, and in the event of for example an interruption of power supply to the control circuit, on reconnection of the control circuit to the power supply the operator will find that he cannot readily select compound or simple steering.